The present invention relates to a method for generating a grating image and, more particularly to a method which can generate a grating image in an improved speed.
In general, a grating image is formed by arranging and combining various micro-gratings. Gratings which have different pitch, orientation and different diffraction efficiencies, will generate various diffraction effects on different wavelength of light. Therefore, it is understood that in order to generate a grating image, it is necessary to control the pitch of each grating, the orientation, the diffraction efficiency and the scale of the gratings as well.
FIG. 1 is a conventional method to generate a grating. A laser beam xe2x80x9caxe2x80x9d and a coherent laser beam xe2x80x9cbxe2x80x9d intersect and form an interferenced standing wave at the intersection area. By placing photosensitive material at the plane Axe2x80x94A, a grating can be formed by recording the interferenced standing waves.
FIG. 2 and FIG. 3 show another method to generate a grating. It can be seen from FIG. 2 that a pair of parallel coherent laser beams xe2x80x9ca1xe2x80x9d and xe2x80x9cb1xe2x80x9d irradiate onto a lens (51) in the vertical direction. After being refracted by the lens (51), the pair of laser beams xe2x80x9ca1xe2x80x9d and xe2x80x9cb1xe2x80x9d intersect at the focus point of the lens (51) and form a standing wave. By placing a photosensitive material (6) at the focal plane of the lens (51), a grating can be obtained by recording the standing wave. If the locations where the laser beams xe2x80x9ca1xe2x80x9d and xe2x80x9cb1xe2x80x9d incidence the lens (51) are adjusted, the pitch and orientation of the gratings will be changed. Alternatively, by adjusting the exposure, the diffraction efficiency of the gratings will be changed. Also, by adjusting the beam size and the divergence angle of the laser beams xe2x80x9ca1xe2x80x9d and xe2x80x9cb1xe2x80x9d the size of the generated gratings will be changed.
FIG. 4 is a previous art for generating a grating image. A laser source (11) is used for sending out a laser beam xe2x80x9cLxe2x80x9d. An acusto-optic modulator (21) (called xe2x80x9cAOMxe2x80x9d hereinafter) is utilized for controlling the exposure. At the beginning, the laser beam xe2x80x9cLxe2x80x9d is emitted from the laser source and passes through the AOM (21) and a beam expander xe2x80x9c3axe2x80x9d. The beam expander xe2x80x9c3axe2x80x9d is composed of the first lens (31) and the first lens (31) or the second lens (32), the divergence angle of the laser beam xe2x80x9cLxe2x80x9d can be adjusted, therefore the size of the generated gratings can be adjusted. The first reflector (111) is located behind the beam expander xe2x80x9c3axe2x80x9d to reflect and change direction of the laser beam xe2x80x9cLxe2x80x9d through the beam expander xe2x80x9caxe2x80x9d. Then the laser beam xe2x80x9cLxe2x80x9d enters a beam splitting and beam guiding device xe2x80x9c4axe2x80x9d. The beam splitting and beam guiding device xe2x80x9c4axe2x80x9d includes a beam splitter (41) which is provided for splitting the laser beam xe2x80x9cLxe2x80x9d into two beams xe2x80x9ca2xe2x80x9d and xe2x80x9cb2xe2x80x9d. The second reflector (42) and the third reflector (44) are arranged within the beam splitting and guiding device xe2x80x9c4axe2x80x9d for changing direction of the beam xe2x80x9ca2xe2x80x9d and allow it to vertically irradiate onto the lens (51A). The fourth reflector (43) is also arranged within the beam splitting and guiding device xe2x80x9c4axe2x80x9d for changing direction of the beam xe2x80x9cb2xe2x80x9d and allow it to vertically irradiate onto the lens (51). After that, the beams xe2x80x9ca2xe2x80x9d and xe2x80x9cb2xe2x80x9d intersect at the focus of the lens (51A) and forms an interferenced standing wave. By placing a photosensitive material (6A) at the focal plane of the lens (51A), the standing wave can be recorded and generate a grating. It is obvious that by moving the beam splitter (41) and the second reflector (42), the distance between two beams xe2x80x9ca2xe2x80x9d and xe2x80x9cb2xe2x80x9d will be changed. Therefore, the pitch of the gratings can be changed accordingly. Alternatively, if the beam splitting and beam guiding device (4a) is rotated along the direction of the incident optical axis of the laser beam xe2x80x9cLxe2x80x9d, the orientation of the generated gratings will be changed. A precise translation stage (71) is provided for supporting and moving the photosensitive material (6A). It can be understood that different grating can be combined upon displacement of the photosensitive material (6) so as to form a grating image. In this method, the AOM (21), the beam expander xe2x80x9c3axe2x80x9d, the beam splitting and beam guiding device xe2x80x9c4axe2x80x9d and the precise translation stage (71) can be controlled by computer and realize automation. The method of using the beam splitting and beam guiding device xe2x80x9c4axe2x80x9d and the lens (51A) is mirrored from the known Laser Doppler Anemometer (hereinafter called xe2x80x9cLDAxe2x80x9d) technology. The speed of generating grating images may be limited due to a slow dynamic response of the beam splitting and guiding device xe2x80x9c4axe2x80x9d and the precise translation stage (71). This is a main shortcoming of the previous arts of generating grating images.
A main object of the present invention is to provide an improved method for generating grating images which is able to greatly increase the speed of the grating image generation by utilizing the technique of beams guiding, symmetrical beams forming and local scan.
In accordance with one aspect of the present invention, a method for generating a grating image comprises a step of generating a coherent beam, a step of controlling exposure of the coherent beam, a step of guiding the coherent beam to a required location, a step of splitting the coherent beam into two symmetrical coherent beams, a step of local scanning, allowing the symmetrical coherent beams to intersect and form an interferenced standing wave at different position in a small local scan area, a photoactive step for recording a grating on a photosensitive material, and a step of changing the position of the the photosensitive material, combining different local scan areas which are full with gratings and forming a large scale grating image.
In accordance with another aspect of the present invention, a system for generating a grating image, comprises a coherent light source for providing a coherent beam xe2x80x9cLxe2x80x9d,
an exposure control device for the coherent beam xe2x80x9cLxe2x80x9d to pass therethrough and control the exposure of the coherent beam xe2x80x9cLxe2x80x9d, a beam guiding device for controlling the pitch and the orientation of each grating, a symmetrical beam generating device for splitting the coherent beam xe2x80x9cLxe2x80x9d into two symmetrical coherent beams, a local scan device for allowing the two coherent beams to intersect and form a standing wave at different position in a small local scan area, a photosensitive material for recording the standing wave, and
a supporting device for supporting and moving the photosensitive material so as to combine different local scan areas which are full with gratings.